Vertex coordinate after pose change
You can use Object.to_mesh() to create a mesh datablock, which has deformations and modifiers applied. Mesh.transform() that with .matrix_world for world space:

import bpy

C = bpy.context
ob = C.object

print(ob.data.vertices[0].co, "- Vert 0 (original)")

me = ob.to_mesh(scene=C.scene, apply_modifiers=True, settings='PREVIEW')
print(me.vertices[0].co, " - Vert 0 (deformed/modified)")

me.transform(ob.matrix_world)
print(me.vertices[0].co, " - Vert 0 (deformed/modified, world space)")

bpy.data.meshes.remove(me)


Note that vertex indices may differ after modifiers are applied (e.g. Array, Bevel, ...).

Alternatively, the bmesh module can be used to achieve the same as the above script.